Imagine taking a bottle of water from a lake, a river, or the open sea. The water may look clear, and you may not see a single animal. Yet inside that bottle could be a small record of life nearby. Fish leave behind mucus, scales, tissue particles, and waste. Other animals leave similar traces. The genetic material in those traces is called environmental DNA, or eDNA. The United States Geological Survey explains that eDNA is DNA released by organisms into the environment, where scientists can collect and study it.